1. Field of the Invention
The present invention relates to an electronic imaging apparatus particularly suitable for obtaining high-definition images.
2. Discussion of Related Art
Electronic imaging apparatuses, including electronic cameras, were limited in image quality by the number of pixels of an electronic image pickup device used. Recently, however, the number of pixels of each electronic image pickup device has been rapidly increasing to such a level that it is possible to expect image quality equal to or higher than that of silver halide photographs. On the other hand, the effective imaging area of electronic image pickup devices is smaller than that of silver halide films. Therefore, there is a demand for an image-forming optical system that shows a high frequency response at a high spatial frequency. However, it is difficult to meet the demand because of the marked degradation of the response due to geometrical-optical factors such as aberrations, and manufacturing errors (e.g. decentration, tolerances, and surface accuracy), together with wave-optical factors such as the influence of diffraction as well as an optical low-pass filter. Further, if there is residual chromatic aberration (secondary spectrum), the loss of color definition is likely to become conspicuous at the edge of a pattern in terms of dynamic range and color saturation. In particular, the influence of secondary spectrum is large; therefore, a vitreous material of high anomalous dispersion must be used a great deal, resulting in a substantial rise in cost. In the case of old cameras for business use (e.g. TV cameras), which had three camera tubes, it was possible to cancel the residual axial chromatic aberrations and lateral chromatic aberrations to a certain extent. However, with the change of image pickup devices from camera tubes to solid-state image pickup devices, it has become impossible to cancel lateral chromatic aberrations. With the change of cameras from three-tube or -chip type cameras to single-chip mosaic filter type cameras, it has become impossible to cancel axial chromatic aberrations. Therefore, it is even more strongly demanded to correct chromatic aberrations remaining in optical systems. Thus, there is an increasing need of removing the residual chromatic aberrations from optical systems in the present state of the art.
The relationship among the focal length fL and field angle 2.omega. of an image-forming optical system of an electronic imaging apparatus and the number of pixels and pixel pitch (distance between the centers of adjacent pixels) of an image pickup device may be approximately given by EQU fL.multidot.tan.omega.=diagonal image height=pixel pitch.times.(2.times.number of pixels).sup.1/2 (1)
It is desirable in order to correct aberration a in the image-forming optical system to satisfy the following condition: EQU fL.multidot.a&lt;pixel pitch.times.2 (2)
Substituting equation (1) into equation (2) and changing the expression gives EQU a&lt;tan .omega./(2.times.number of pixels).sup.1/2 (3)
It should be noted that in the above expressions, a is axial chromatic aberration or lateral chromatic aberration when fL is set equal to 1 (fL=1) at a certain F-number.
As will be understood from the above, the target chromatic aberration is seemingly dependent only on the number of pixels. However, as the pixel pitch decreases, the influence of various response-reducing factors becomes remarkable, as stated above. Therefore, when an optical system is designed, the target aberration a must be made smaller. Under these circumstances, attention has recently been paid to diffraction optical elements that exhibit optically superior correcting capability for higher-order spectra in particular, and studies have been conducted to put diffraction optical elements to practical use. In the meantime, diffraction optical elements have a problem which is difficult to resolve. The problem is that, when it is intended to obtain a specific order of diffracted light over a wavelength width needed for an electronic imaging apparatus, other orders of diffracted light unavoidably get mixed in the desired order of diffracted light. The undesirably mixed diffracted light is referred to as "unwanted orders of diffracted light", which cause flare and degrade image quality.